This invention relates to X-ray apparatus. More particularly, it is concerned with apparatus in which the source of X-rays is a metal target bombarded with an electron beam.
An essential element of any X-ray radiographic imaging system is a source of X-ray radiation. One common technique for providing this source is by bombarding a metal target with a beam of electrons in a high vacuum environment. The size of the spot where the electron beam strikes the target affects the resolution or clarity observed in a film image of an object exposed to the X-ray radiation. The smaller the spot, the sharper the resulting image. If the spot moves due to any of a number of factors while the X-ray film is being exposed, the resulting image suffers some distortion.
Many X-ray systems have focal spots ranging from 0.4 millimeters to 3 millimeters in diameter. Slight motion of the spot, of the order of tens of micrometers, does not significantly affect the clarity or sharpness of a radiographic image because this relatively large spot causes a predominant blurring or unsharpness that masks the effect of this motion. The effects of the motion of the X-ray focal spot, however, are more noticeable when the spot is smaller. Presently available microfocus X-ray systems produce a focal spot on the order of 10 micrometers in diameter. Motion of such a focal spot during the period of an exposure, even if the motion is less than 10 micrometers, can be a serious problem. The problem is especially acute when long exposures are required or image magnification is employed or tomographic imaging (CAT scanning) is involved. It is, therefore, desirable to maintain the X-ray focal spot relatively stationary with respect to the target during the period of exposure in order to eliminate motion-induced distortion.